Tack Coat
How Tack Coat Improves Your Asphalt Paving
Inexpensive tack coat can significantly improve the bond between pavement layers, improving pavement strength, reducing surface-course sliding and reducing top-down cracking
Application of tack coat to an existing pavement surface is so simple and inexpensive that its real importance is often overlooked. Normally considered an incidental expense tack-coat material cost is often not paid for directly but rather included in the cost of other contract items. But proper application of tack coat is critical to the performance of a hot mix asphalt (HMA) pavement structure.
The proper application of asphalt tack coat can significantly improve the bond between the pavement layers, increasing the strength of the pavement structure. Tack coat can also reduce the tendency for the surface course layer to slide on the underlying layer under the force of accelerating or decelerating traffic. Finally, tack coat can reduce the occurrence of top-down cracking in the surface course layers.
When is a Tack Coat Needed?
A tack coat is normally applied to an existing pavement surface before a new layer of HMA is placed. This is done whether the existing surface is an old asphalt concrete layer or a portland cement concrete (PCC) pavement. In some cases, a tack coat is applied to a new HMA pavement layer before the next layer is placed–such as between a HMA surface course constructed on a HMA leveling course.
Many people believe that a tack coat is needed in order to provide a “bond” between an existing pavement surface and a new HMA layer. Some people believe that a tack coat is needed to prevent the new HMA layer from sliding on the existing pavement layer. When, and to what extent, the bond is formed between layers, however, is the subject of great debate.
Whether the tack coat performs as expected, however, depends on a great number of factors. Among those factors is the type of asphalt material used for the tack coat, such as bitumen emulsion or asphalt binder. Regardless of the type of material used, however, it is not the application rate of the tack coat material that is important, it is the residual amount of asphalt binder remaining on the pavement surface once the tack coat material has “set.”
The proper amount of residual tack coat material on the existing pavement surface normally does not cover the entire surface. In the vast majority of paving situations, the tack coat is distributed over only 90 to 95 percent of the existing surface. This means that 5 to 10 percent of the existing surface is still visible once the tack coat has been applied. Too little tack coat might result in a significant reduction in the strength and durability of the pavement structure. Too much tack coat, however, can easily create a slip plane between the layers and cause the upper pavement layer to slide on the lower layer.
It has often been found, when cores are cut shortly after HMA pavement layers are constructed, that bond between the pavement layers has not occurred. Many variables affect the rate of bond development: climatic factors, the amount of traffic, the thickness of the layers, and the degree of density obtained in the HMA mixture. In some cases, the bond between the new asphalt layer and the old, existing pavement might occur within a few days or weeks. In other cases, however, it could be many weeks or even months before the bond is developed between two layers.
Cores cut from a new overlay, then, might not be immediately bonded to the old pavement surface. It has been found that the bond will develop with time, traffic and warm weather. The bond will still eventually develop even though only 90 to 95 percent of the existing pavement surface is covered with the tack coat material.
There is no doubt that the application of the tack coat material aids the bond development between the pavement layers and thus the transfer of the traffic loads throughout the thickness of the pavement structure. The tack coat also greatly reduces the amount of sliding or slippage failures that may occur between the existing pavement surface and the new HMA pavement layer. Thus, the proper application of the tack coat is very important in the long-term durability of the asphalt concrete pavement.
Selecting Tack Coat Material
In the past, the most widely used type of asphalt material used for a tack coat application was a cutback-type material such as RC 800, RC 3000, or MC 3000. Due to environmental concerns, however, cutback asphalts are normally not used for tack coat applications today.
The most common type of asphalt material currently in use for tack coat applications is an asphalt emulsion. A number of different grades of the emulsion are used including both rapid set and slow set type materials. For the rapid set emulsions, usually, an RS-1, RS-1h, CRS-1 or a CRS-1h material is specified. For the slow set emulsions, the most common grades are SS-1, SS-1h, CSS-1, and CSS-1h. For some high-traffic roadway paving projects, the asphalt emulsion may be polymer modified.
Detailed information on the properties and characteristics of the various grades of asphalt emulsion can be found in the American Society for Testing and Materials (ASTM) Specification D 997 for emulsified asphalt and D 2397 for cationic emulsified asphalt. Research has shown that the use of a hard base asphalt binder in the emulsion (use of SS-1h in lieu of SS-1) sometimes results in an increase in bond strength and a reduction in the occurrence of sliding failures.
An asphalt emulsion is a combination of asphalt binder (asphalt cement), water, and a very small amount of emulsifying agent (normally less than 1 percent). Typical asphalt emulsions consist of 55 to 70 percent asphalt binder, depending on the grade of the emulsion. Most grades of asphalt emulsion used for tack coat applications contain approximately 60 to 65 percent asphalt binder. When the water in the emulsion evaporates, it is the residual asphalt content in the emulsion that remains on the pavement surface. For ease of calculation purposes, an asphalt emulsion commonly used for a tack coat application can be taken to be approximately two-thirds asphalt binder (67 percent asphalt binder) and one-third water (33 percent water).
In some areas, asphalt binder is used for the tack coat material instead of asphalt emulsion. In this case, the tack coat material consists of 100 percent asphalt material. The grade of asphalt binder selected typically is the same as incorporated into the HMA mix. For most areas, the asphalt binder tack coat material would meet the requirements of a performance graded (PG) binder–for example, a PG 64-22 or PG 58-28. In some instances, the asphalt binder may be polymer modified.
How much Tack Coat Do You Need?
The amount of tack coat actually needed is a function of a number of factors. It is not correct to apply the same amount of tack coat material to all pavement surfaces regardless of the condition of that surface. Typical residual asphalt application rates and actual application rates are shown in Table 1.
The primary factor to be considered is the type and condition of the existing pavement surface. If that surface is relatively new, clean, and smooth, a minimum amount of tack coat is normally needed. This is due to the fact that little tack material will be absorbed into the existing surface and minimal amount of tack material will be needed to offset the dust on the surface. For a clean, tight surface the residual amount of tack coat required will generally be in the range of 0.02 to 0.03 gallons per square yard (gal./sq. yd.).
If the existing pavement surface is aged and oxidized, the residual amount of tack coat applied should be increased slightly to compensate for any material that might be absorbed into the old surface. Further, if the existing pavement surface contains many hairline cracks, the residual amount of the tack coat material should also be increased slightly. For an aged, oxidized existing surface, the residual amount of tack coat required will generally be in the range of 0.04 to 0.06 gal./sq. yd.
If a new leveling course has been placed over the underlying asphalt or concrete pavement surface, it might not be necessary to apply a tack coat on the new leveling course. If, for example, an HMA leveling course is placed one day and the HMA surface course is placed over the leveling course the next day, a tack coat typically is not needed. If the leveling course layer is clean and smooth there will be minimal absorption of the tack coat material. If, however, the leveling course has gotten dirty between the time of its placement and the time of the placement of the next HMA course, the pavement surface should be cleaned and a minimum amount of tack coat applied. In the latter case, the residual amount of tack coat material should be in the range of 0.02 to 0.03 gal./sq. yd.
Some recent research has found that the application of a tack coat between new layers of HMA is beneficial in reducing the occurrence of top-down cracking in the new layers. In addition, tack coat between new layers of HMA should be required in locations of accelerating or decelerating traffic, turning areas, steep grades, high volumes of traffic, very hot climates and between relatively thin layers of HMA.
In the case where the original pavement surface, either HMA or PCC has been milled, the tack coat application rate should be increased to compensate for the dust from the milling operation and the increase in the surface area and surface texture of the milled surface. Even though the milled surface has been cleaned by brooming or flushing with high-pressure water, some dust, dirt, and debris might remain in the bottom of the milling grooves. For this type of construction operation, the residual amount of tack coat material should be in the range of 0.05 to 0.07 gal./sq. yd. Some individuals believe that a tack coat does not need to be applied to a milled pavement since the rough, milled surface texture will prevent the new pavement layers from sliding. It is strongly suggested, however, that a tack coat be applied to the milled surface to help achieve a bond between the new and the old pavement layers.
Finally, if the pavement layer being resurfaced is portland cement concrete, it might be necessary to slightly increase (rougher surface) or decrease (smoother surface) the residual tack coat amount depending on the texture of the PCC surface. A relatively smooth surface texture will require less tack coat and a relatively rough surface texture will require more tack coat. The residual amount of tack coat required will generally be in the range of 0.04 to 0.06 gal./sq. yd.
A secondary consideration is the type of terrain where the pavement is located. If the grade is relatively flat, the residual tack coat application rates given above are typically adequate. However, if the roadway being paved is located in a hilly or mountainous area, it might be prudent to reduce the amount of tack coat. Lowering the residual tack coat rate by 0.01 gal./sq. yd. or keeping the residual application rate at the lower side of the suggested range would be recommended. It is very important to note that excessive tack coat on the pavement surface in mountainous terrain might cause problems with slippage of the haul trucks and the paver in the tack coat when moving up or down hill and slippage of the completed pavement course at the interface between the layers. The reduction in the rate of the residual tack coat application due to the above construction considerations in hilly terrain is offset, however, by the possible reduced performance of the pavement layers due to the high shear stresses caused by traffic moving uphill or downhill.
Another secondary factor is the environmental conditions at the time of paving. If an asphalt emulsion is used for the tack coat, and if the humidity is very high or the ambient temperature is very high, the emulsion setting and curing time will be extended. The same is true if the pavement surface is damp from the rain. Reducing the residual amount of tack coat by 0.01 gal./sq. yd. or keeping the residual application rate at the lower side of the suggested range would again be prudent. A change in the type of tack coat material used, from a soft asphalt emulsion to a hard asphalt emulsion (SS-1 to SS-1h, for example) should be considered. A change from an asphalt emulsion to an asphalt binder might also be feasible.
How to Apply Tack Coat
Improper application of tack coat, like this tack that was applied by hand using a pour bucket, eliminates any benefits that can result from using tack coats to improve paving.
To determine the application rate for an asphalt emulsion tack coat, it is absolutely necessary to start with the residual asphalt content – the amount of asphalt binder remaining on the pavement surface once the water in the emulsion evaporates. If, for example, it has been determined that a residual asphalt content of 0.04 gallons per square yard (gal. /sq. yd.) is needed, an application rate of 0.06 gal. /sq. yd. should be delivered from the asphalt distributor. This calculation is based on a ratio of two-thirds residual asphalt binder and one-third water in the asphalt emulsion and is done by multiplying the residual asphalt content by a factor of 1.5 to determine the required application rate (as discussed in “How a Tack Coat Improves Your Paving,” in the May issue of Pavement Maintenance & Reconstruction).
If the residual asphalt content of the asphalt emulsion was required to be 0.06 gal. /sq. yd., for example, the emulsion application rate from the distributor would be approximately 1.5 times as great or 0.09 gal. /sq. yd. (0.06 x 1.5 = 0.09). Similarly, if the required residual asphalt content was 0.05 gal. /sq. yd., the application rate from the distributor would be 1.5 times as great, or 0.075 gal. /sq. yd.
In some cases, it is advantageous to dilute an asphalt emulsion in order to achieve a more uniform application of the residual asphalt binder in the emulsion on the pavement surface. The usual dilution ratio is 1:1 – one part water is added to one part emulsion. This means that the original emulsion makes up only 50 percent of the diluted emulsion. It also means that only one-third of the diluted emulsion will actually be residual asphalt binder – two-thirds will now consist of water. The water used to dilute the emulsion must be potable – suitable for drinking.
It should be noted that only slow setting emulsions can be diluted with water. When a slow set emulsion is diluted, the water is added to the emulsion – not the emulsion to the water. Adding the emulsion to the water might cause the emulsion to break and cause water and the asphalt binder to separate. A diluted emulsion will typically take longer to set than will an undiluted emulsion under the same environmental conditions, thus possibly delaying the placement of the hot mix asphalt mix.
For an emulsion that is diluted 1:1 with additional water, for a required residual asphalt content of 0.04 gal. /sq. yd., it would be necessary to apply the diluted asphalt emulsion at a rate of 0.12 gal. /sq. yd. from the asphalt distributor. This application rate is calculated by multiplying the residual asphalt content by a factor of 3.0 to determine the required application rate of the 1:1 diluted emulsion.
If the residual asphalt content was required to be 0.06 gal. /sq. yd., for example, the application rate of the diluted emulsion would be approximately 3 times as great, or 0.18 gal. /sq. yd. (0.06 x 3.0 = 0.18). Similarly, if the required residual asphalt content was 0.05 gal. /sq. yd., the application rate of the diluted emulsion from the distributor would be 3 times as much, or 0.15 gal. /sq. yd.
It is very important to note that it is the residual asphalt content in the emulsion that is the key to the tack coat performance. If a distributor operator applies an undiluted asphalt emulsion tack coat at the rate of 0.10 gal. /sq. yd., the amount of residual asphalt will be approximately 0.066 gal. /sq. yd. If a distributor operator applies an asphalt emulsion that has been diluted 1:1 with water at the same application rate, 0.10 gal. /sq. yd., however, the amount of residual asphalt will be only 0.033 gal. /sq. yd. This reduced residual amount most likely will not provide adequate performance. The application rate of the tack coat should always be calculated by starting with the required residual rate and working backward to determine the application rate.
The distributor operator must know the type of material being used and, if an asphalt emulsion is being used, whether the emulsion has been diluted or not. In addition, he or she must know what the residual asphalt content is supposed to be and then back-calculate to determine the application rate of the emulsion from the distributor. Unfortunately, many distributor operators simply do not know whether the emulsion has been diluted or not or know how to determine the application rate and/or the residual rate for the tack coat material. If pure asphalt binder is used for the tack coat, the application rate from the distributor spray bar and the residual rate on the pavement surface will be exactly the same.
Asphalt Application Temperature
Proper tack coat application begins with the asphalt distributor (shown in Figure 1) used to deliver the tack coat material to the existing pavement surface. The tack coat material in the distributor tank must be kept at the proper temperature to assure that the material can be sprayed uniformly onto the existing pavement surface. When an asphalt emulsion is used, it should normally be applied at a temperature between 120 F and 160 F. When an asphalt binder is used, it should normally be applied at a temperature between 275 F and 325 F. It is very important to have the tack coat material at the proper application temperature in order to obtain a uniform distribution on the existing pavement surface.
Spray Bar Nozzle Size
Uniform tack coat application also depends on the size of the nozzles used on the distributor spray bar. If asphalt binder is used and the application rate (and residual amount) is only 0.04 gal. /sq. yd., for example, the nozzle size used should be appropriate to uniformly apply that amount of material. If a 1:1 diluted asphalt emulsion is used and the application rate is 0.18 gal. /sq. yd. (for a residual amount of 0.06 gal. /sq. yd.), it will generally be necessary to use larger nozzles on the spray bar to achieve the desired tack coat uniformity.
It is noted that the nozzle size needed to apply an asphalt emulsion for a surface treatment or chip seal is significantly larger than the nozzle size required to apply an asphalt emulsion for a tack coat. For a surface treatment which requires a residual asphalt content of 0.28 gal. /sq. yd. to properly hold the cover aggregate, the application rate for an undiluted emulsion would be 0.42 gal. /sq. yd. Compare that application rate to that of an undiluted asphalt emulsion rate of 0.06 gal. /sq. yd. for a tack coat with a residual asphalt content of 0.04 gal. /sq. yd. Obviously, the same size nozzles cannot be used for both types of application.
A chart usually supplied with each distributor provides the operator with information on the proper nozzle size to use for different application rates. In too many cases, the same distributor used for surface treatment construction is used to apply the tack coat material during a hot mix asphalt paving operation. If the nozzles are not changed, the asphalt tack coat application will be extremely non-uniform – the asphalt material will come out in longitudinal streaks instead of a fan-like spray. The pump pressure on the distributor must be set to match the desired application rate of the tack coat.
Spray Bar Operation
Once the correct nozzle size is selected for the desired application rate and the type of tack coat material being applied, the next step is to assure that all of the nozzles on the spray bar are set at the correct angle and are functioning properly. This means that the nozzle opening (slot) should be set at an angle of approximately 30 degrees to the axis of the spray bar. If all of the nozzles are not set at the same angle, non-uniform application of the tack coat will result. In addition, all the nozzles must be working correctly. If a nozzle is partially or completely blocked, non-uniform application of the tack coat material will result.
For a tack coat application, a double or triple lap of the asphalt material from the spray bar to the road surface can be used. The amount of overlap will depend on the height of the spray bar above the pavement. In most cases, a triple lap is used even though it is not necessary to completely cover the existing pavement surface with the tack coat – 90 to 95 percent coverage is typically adequate.
As the tack coat material in the tank on the distributor is used, the height of the spray bar might rise slightly, changing the amount of nozzle overlap. So the height of the spray bar must be manually adjusted to compensate for the change in the height of the bar as the distributor is emptied. Perhaps the simplest approach is to set the bar height when the tank is half full.
The forward travel speed of the distributor must be controlled to assure that the application rate is correct. If the distributor travels too fast, the amount of tack coat applied to the pavement surface will be reduced. The opposite effect occurs if the distributor travels too slowly for a given spray bar nozzle size and pump pressure. Luckily today’s computer-controlled distributors manage variations in speed and are able to maintain the application rate while speeding up or slowing down.
Conducting a Trial Tack Coat
Before starting the tack coat application, the distributor operator should check the temperature of the asphalt material to assure that it is at the correct spraying temperature for the type of material being used. In addition, the nozzles on the spray bar should be checked to assure that they are of the proper size and that they are all turned to the same angle to the axis of the bar.
If the distributor has not been used for a time, it is good practice to construct a trial placement of the tack coat over some convenient, unused area to assure that all the nozzles are operating properly. In addition, the trial run can be used to judge the tack coat application uniformity as well as to confirm the proper amount of tack coat application. If desired, the distributor application rate can be calibrated, both in a transverse direction and in a longitudinal direction, in accordance with ASTM D 2995.
How to Prepare the Pavement Surface
If the existing pavement surface is dusty, the application of the tack coat may not provide the adequate bond between the pavement layers or prevent sliding of the new layer on the old layer. Thus it is extremely important to thoroughly clean the existing surface before the tack coat is applied. This should be accomplished by properly sweeping and/or flushing the existing pavement surface with high-pressure water to remove any dust film before placing the new HMA material.
Why Tack Coat Improve Parking Lots and Driveways
Tack and multiple layers extend pavement life, improve investment return.
Asphalt emulsion bond coat placed full width with overlapping nozzle pattern will result in more durable parking area overlay.
Parking lots and driveways don’t have to be an afterthought to a new commercial or residential construction project.
But at times they are. To save money on new construction, an owner may insist on a 2-in. asphalt overlay over granular base, rationalizing that only passenger cars are going to be using the drives and parking areas.
This rationale is blown out of the water as refuse trucks right away begin to visit the concrete dumpster pad, and ruts promptly appear leading up to it. The ruts lead to cracking, which allows ingress of water into the base, leading to late winter potholes.
Regrettably, that’s only the beginning of paved surface headaches for the owner.
An answer is thicker, stronger, multiple lifts of asphalt on granular base, or fresh asphalt overlays on existing pavements, both of which provide the aesthetic look that gives customers confidence in doing business with commercial or professional interests located in the facility.
“Pavements in parking areas that are initially under designed can experience excessive maintenance problems and a shortened service life,” reports the Colorado Asphalt Pavement Association. “In addition, selecting the right materials for the asphalt pavement can assure a pleasing and attractive surface.”
But long-term cost effectiveness can be an even more important driver than aesthetic pleasantness, CAPA says. “When properly designed and constructed, parking areas can be an attractive part of the facility that is also safe, and most important, usable to the maximum degree. Parking areas should be designed for low maintenance costs and easy modification for changes in use patterns.”
Multiple Lifts Save Money
Ideally, asphalt pavements for commercial parking areas or drives typically are placed in “lifts,” or layers, for a total compacted thickness of 3 or more in.
“For example, a total of 3 in. may be placed in two layers each 1.5 in. thick when compacted,” explains the National Asphalt Pavement Association. “A good way to pave a 4-in. pavement is to first place about 3 to 4 in. of loose asphalt, and compact it to a thickness of 2.25 to 3 in., and then tack the surface and pave the remainder and compact again. In this case, different mixes could be used in each layer, such as a mix with larger stones (12.5 mm nominal maximum aggregate size, NMAS) for the lower layer and smaller stones (9.5 or 4.75 mm NMAS) for the surface layer. The finer surface mix provides a uniform, attractive, impermeable, and strong surface.”
For typical residential driveways, an aggregate base layer of 6 to 8 in. of compacted crushed aggregate is recommended by the Asphalt Institute on top of a stable, prepared subgrade. This course is followed by approximately 2 in. compacted of dense-graded HMA base and 1.5 in. compacted HMA surface mixes.
While project owners may want to skimp on the depth of asphalt of their parking lot or drives, that extra inch or two of asphalt provides owner benefits way beyond the initial cost, says Jim Huddleston, executive director of the Asphalt Pavement Association of Oregon (OPAO).
Huddleston explains that adding one inch of asphalt to the pavement can double the fatigue life of the structure, whether a state highway, county road, municipal street, or commercial pavement or driveway.
“If a 3-in. pavement is expected to last 20 years, for example, adding one inch of asphalt will increase the fatigue life to 40 years,” Huddleston says. “Adding another inch will double the fatigue life again (to 80 years). A road need only have an asphalt thickness of 5 to 6 in. in order to have perpetual properties (such as cracking and rutting developing at the surface rather than bottom-up).”
While 5 to 6 in. of asphalt may go beyond the needs of commercial facility owners, for a new construction project, adding one more inch of asphalt typically amounts to no more than the cost of the material itself and delivery to the site, Huddleston says.
“It’s the cheapest inch of asphalt you’ll ever buy,” he said, as the life-doubling benefits of an extra inch of asphalt can be reaped for both new pavements under construction as well as existing pavements that have already held traffic.
Emulsions for Bond Coats
Whether you are placing two or more lifts of asphalt on a parking area, or overlaying an existing driveway or parking surface, the rule remains the same: for durable pavements, bond the layers of asphalt with a bond coat, also called a “tack” coat.
An asphalt emulsion is a homogeneous mixture of two insoluble substances, oil and water. In it particles of liquid asphalt (the dispersed phase) are surrounded by molecules of water (the continuous phase). Compared to hot liquid asphalt, asphalt emulsions have greatly reduced viscosity, are safe to use at lower temperatures, and allow liquid asphalt to be spread more evenly and thinly.
Emulsified asphalt is produced by dispersing tiny globules of asphalt cement into water treated with a small quantity of emulsifying agent. The dispersion takes place in a powerful blender, called a colloid mill, where spinning blades break or shear the liquid asphalt into suspended microscopic particles. The water, or soap solution, is immediately introduced to form the emulsion.
The emulsifier – an engineered surfactant (detergent) or surface-active agent – maintains the microscopic asphalt droplets in a stable suspension, keeping them from recombining. The amount and type of surfactant used, along with other variables, controls properties of the emulsion critical to performance in the field application.
For example, emulsifiers affect the “break” time following placement on a road, in which the water evaporates, leaving the residual asphalt behind. Asphalt emulsions partially are categorized as the rapid setting (RS), medium setting (MS), slow setting (SS) and quick setting (QS) emulsions.
Today, the most common types of emulsions used for bond coats include slow-setting grades of the emulsion such as SS-1, SS-1h, CSS-1, and CSS-1h and the rapid-setting grades of the emulsion such as RS-1, RS-2, and CRS-1.
Bonding Asphalt Layers
Bond coats utilize either straight liquid asphalt, cutback asphalt, or asphalt emulsions, but for a number of reasons, asphalt emulsions constitute the lion’s share of bond coat applications by far, according to National Cooperative Highway Research Council (NCHRP) Report 712, Optimization of Tack Coat for HMA Placement (for your copy, search “NCHRP Report 712”).
“The most widely used tack coat material in the world is emulsified asphalt,” NCHRP reports, adding emulsified asphalt, or asphalt emulsion is a nonflammable liquid substance that is produced by combining asphalt and water with an emulsifying agent. A survey in the report indicated that worldwide, approximately 92 percent of tack coat applications utilized asphalt emulsions.
It’s easy to see why asphalt emulsions are preferred for bond coats. Straight or “neat” liquid asphalt must be kept heated to stay liquefied, and that poses burn hazards and other safety problems as the distributor moves around on the site.
An asphalt emulsion bond coat, when sprayed onto the pavement, functions as a glue to hold existing and new pavement layers together. It enhances the bonding of individual pavement layers so they function as a single, unified pavement for improved strength and durability. When properly bonded, the resulting pavement structure uniformly distributes traffic load stress. Without proper bonding, each pavement layer works independently, failing to distribute stress evenly, leading to cracking, rutting, slippage and other distresses that destroy road quality.
In the same way that fragile, thin veneers of wood are glued to each other to form a robust sheet of plywood, research shows layers of asphalt pavement perform better when bonded to each other.
“An emulsified asphalt tack coat produces a strong adhesive bond without slippage between an existing pavement and a new overlay,” reports the Colorado Asphalt Pavement Association in its 2013 report, Best Practices for Applying Undiluted Emulsified Asphalt Tack Coat. But the performance of this adhesive bond can be endangered by poor placements.
Yet best practices are well established. For example, for maximum service, bond coats should be evenly distributed across the full width of clean pavement, not just as the commonly seen narrow spritzes on the substrate. Spray bar nozzles must overlap, ideally in multiples of three; triple overlap is recommended for higher application rates such as chip seal, but may not be achievable for lower rates of bond coats.
The emulsion must be dispersed with the right spray pattern, at the right temperature and volume. Long after the paving is completed, inadequate adhesion between two layers of bituminous mix may cause problems in the mat above.
An asphalt overlay will make an older driveway or parking area look new, so long as the existing pavement is sound structurally. The contractor should repair any potholes, cracks, or areas which have been softened due to spills or drips of petroleum products. Alligatored areas, widened joints and cracks certainly will reflect through the new overlay, and must be repaired or mitigated prior to overlay.
A complete brooming — followed by a complete asphalt emulsion bond coat spray — will adhere the new 1.5– to 2-in. overlay to the substrate.
“If [the overlay] can’t stick it’s not going to bond,” said consulting engineer Dale Decker, P.E., at a seminar on tack coats during World of Asphalt 2015 in Baltimore. “And if we don’t get the bond, we’re wasting our money.”
Moreover, from the owner and contractor point of view, this bonding of layers via asphalt emulsions comes at a relatively small price.
“An analysis of bid tabs using 2013 data shows that the cost of tack coat is relatively minor compared to other components of a typical paving project,” Decker said. “Therefore, the risk of a poor or a compromised bond from not applying tack should be deemed too great when one considers the ramifications of the loss of fatigue life from this condition. Tack is such a cheap component that it makes little sense to shortchange this critical step and introduce so much risk.”
